An efficient way to optimize a power amplifier (PA) current consumption in a wireless system across an entire output power range is the use of a DCDC (DCDC=direct current to direct current) converter which provides a power amplifier supply voltage. Depending on the output power, the output voltage of the DCDC converter is adjusted. The lower the output power is, the lower is the required power amplifier supply voltage. Due to the voltage conversion from the battery voltage down to the lower power amplifier supply voltage, the battery current is reduced. Conventionally, the DCDC converter output voltage is set based on a target power (average power) which is expected in a next slot. This procedure is called average power tracking (APT).
In the future, fast DCDC converters, so called envelope tracking (ET) DCDC converters, will further reduce the battery current. Envelope tracking describes an approach for RF amplifier design in which the power supply voltage applied to the power amplifier is constantly adjusted to ensure that the amplifier is operating at or close to peak efficiency for the given instantaneous output power requirements.
A feature of envelope tracking is that the supply voltage of the power amplifier is not constant. The supply voltage of the power amplifier depends on the instantaneous envelope of the modulated baseband signal or RF (RF=radio frequency) input signal, input into the power amplifier. In a highly simplified description the envelope of the modulated baseband signal is calculated by means of the CORDIC (CORDIC=coordinate rotation digital computer) algorithm, followed by a delay adjustment to compensate for a different delay in the main signal path (RF signal generation path) and envelope path, then the envelope signal is shaped (pre-distorted) and finally digital-to-analog converted. This signal is applied to the envelope tracking DCDC converter (special ultra fast DCDC converter) which generates the variable power amplifier supply voltage.
An envelope tracking capable DCDC converter follows the instantaneous envelope of the RF signal which removes the voltage headroom and further increases the system efficiency (=composite efficiency of the power amplifier and the DCDC converter). It is expected that an envelope tracking capable DCDC converter will reduce the battery current of a LTE (LTE=Long Term Evolution) signal by roughly 20+% at maximum output power relative to a standard DCDC converter which simply follows the average power.
To allow envelope tracking operation and to maximize the efficiency enhancement in an envelope tracking system the power amplifier must be designed in a different way compared to a traditional power amplifier design optimized for average power tracking. This must be done to account for the envelope tracking specific requirements.